As the technical development and the demand for mobile devices increase, the demand for secondary batteries as an energy source has been rapidly increasing. Among secondary batteries, lithium secondary batteries having a high energy density and voltage have been commercialized and widely used.
Generally, a lithium secondary battery may be prepared using a positive electrode and a negative electrode that include an electrode active material which may intercalate and deintercalate lithium ions, and an electrolyte which is a delivery medium of lithium ions.
Conventionally, an electrolyte in a liquid state, particularly an ion conductive organic liquid electrolyte obtained by dissolving a salt in a non-aqueous organic solvent, has been widely used as an electrolyte. However, a liquid electrolyte may lead to leakage and ignition and explosion due to the high flammability of the non-aqueous organic solvent used. In addition, with the liquid electrolyte, a carbonate organic solvent may be decomposed while charging and discharging a lithium secondary battery, or a side reaction with an electrode may occur that generates a gas. The side reaction may be further accelerated during storage at a high temperature, increasing the amount of the gas generated.
The continuously gas generated causes an increase in internal pressure of a battery, thereby not only causing deformation of the battery such as expanding its thickness, but also causing local differences in adhesion at the electrode surface of the battery, resulting in the problem that the electrode reaction does not occur uniformly on the entire electrode surface.
Thus, in order to solve this stability problem of a liquid electrolyte, a method of using a gel polymer electrolyte which is free from leakage and the like has recently been proposed.
The gel polymer electrolyte may be prepared by impregnating a polymer matrix formed by the polymerization reaction of a polymerizable monomer and a polymerization initiator with an electrolyte containing an electrolyte salt and an electrolyte solvent, and then gelling the mixture.
However, the gel polymer electrolyte is disadvantageous in that the lithium ion conductivity is lower than that of the liquid electrolyte formed solely of an electrolyte. In order to solve this problem, a method of reducing the thickness of the gel polymer electrolyte has been proposed, but in this case, since mechanical strength is reduced and the gel polymer electrolyte is short-circuited during the production of the battery, there is a problem in the performance and safety of the secondary battery.
Therefore, there is a need for the development of a gel polymer electrolyte having improved battery performance such as ion conductivity, mechanical strength, and the like.